In lithographic printing, printing is conducted using a printing plate having ink receptive image areas and hydrophilic non-image areas. Printing is conducted in such a manner that a dampening solution is applied to the hydrophilic surface of the printing plate. When ink is applied, the solution is retained by the hydrophilic areas, but repelled by the ink-receptive printing areas. It is important that the ink and the dampening solution are fed to the surface of the plate with a good ink-water balance.
When there is too much dampening solution, the ink is excessively emulsified by the solution drying is retarded and offset is caused. When there is too little dampening solution, the ink adheres to the non-image areas and scumming results.
Dampening solutions were initially used to prevent scumming during printing. Printing engineers often prepared the solutions using chromates, optionally together with metaphosphoric acid or gum arabic. With improvements in printing quality and printing workability, various types of water feed systems have been developed. As a results, the requirements of dampening solutions have changed. Dampening solutions are required to not only remove scum, but also provide various often performance characteristics. At the same time, dampening solutions must effectively address environmental concerns.
The dampening system invented by Dahlgren in 1960 was an epochmaking invention. The system was introduced into Japan in about 1965, and for the first time an aqueous solution containing IPA (isopropyl alcohol) was used as a dampening solution. At first, dampening solutions contained about 25% IPA. However, this amount was gradually reduced to 5 to 15% due to printability and environmental problems.
When IPA was included in dampening solutions, it was found that the solutions could be used in the form of a thin film, and that it provided faster processing, an improvement in printing quality, and enhanced automation. Thereafter, various continuous dampening systems were developed in succession by domestic and foreign printing press manufactures.
Most of the subsequently developed continuous dampening systems were not inker feed systems like Dahlgren's system where the dampening solution was fed using inked rollers. Instead, they were plate feed-type dampening systems where the dampening solution was fed using rubber rollers independent of the inked rollers. These subsequently developed dampening systems differed from one another in the type of roller materials, the number of rollers, the construction of rollers, the presence or absence of reverse slip nip, the presence or absence of rider rollers and the presence or absence of delivery rollers between inked rollers and dampening rollers.
The aforementioned dampening systems were designed to be used with IPA. The characteristics of the continuous dampening systems were effectively utilized using IPA. That is, a minimum amount of the dampening solution was uniformly applied, and the dampening solution was quickly stabilized so that rising is rapid providing reduced waste and spoilage.
The use of the dampening solutions containing IPA and the continuous dampening systems were, and are still popular. There are, however, Sleeve and Molton systems. In these systems, the dampening solution can be metered and good prints can be obtained. But, when IPA is used in the dampening solutions, there are certain problems.
The first problem is a environmental one. Handling is restricted by certain labor safety hygiene laws, fire laws and sewage laws in Japan.
There are labor safety hygiene laws (organic solvent poisoning prevention rules), which apply to dampening solutions containing at least 5% of IPA. For example, a local evacuation system must be provided when dampening solutions contain at least 5% of IPA are used.
Furthermore, here are rules which require users to make certain environmental measurements and undergo medical examinations.
Unfortunately, when the concentration of IPA is less than 5%, users have been unable to obtain a satisfactory printing effect. And continuous dampening systems developed for rapid processing have likewise been unsuitable.
In Japan, IPA is designated as a fourth petroleum alcohol and must be handled in the same manner as gasoline. IPA is regulated by certain fire laws when stored and used.
Certain sewage laws require users to provide treatment facilities when the pH of the waste is less than 5, or more than 9, or when the biochemical oxygen demand (BOD) is 600 mg/l or higher. IPA relates to the latter. With IPA waste both the pH and BOD must be controlled.
Another problem associated with the use of IPA, is cost. In perfecting four-color web-offset printing presses equipped with continuous dampening system, there are large amounts of IPA used, and cost is high.
To improve the problems associated with IPA, there is proposed in JP-B-61-55480 (the term "JP-B" as used herein means an "examined Japanese patent publication") that the content of the alcohol is reduced to 10% by weight or below. However, the problems can not be solved by only reducing the content of JPA. That is, the amount of IPA is still too large, because to avoid regulation by the organic solvent poisoning prevention rules the amount of IPA should be not higher than 5% or less by weight.
The problem of unstable concentration due to evaporation of IPA can not be effectively addressed so long as dampening solutions contain IPA. U.S. Pat. No. 3,877,372 addresses the problem of the volatility of volatile alcohols, by using butyl cellosolve rather than volatile alcohols. U.S. Pat. No. 4,278,467 describes a mixture of two or more components. These U.S. Patents describes dampening solutions which contain nonionic compounds and are free from IPA.
JP-A-57-199693 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describe dampening solutions containing 2-etyl-1,3-hexanediol.
Unfortunately, when dampening solutions containing no volatile alcohols are used on printing presses equipped with continuous dampening systems (e.g., the Dahlgren system) to conduct printing, there are several disadvantages. For example, ink adheres to the surface of metering rollers to cause tinting and a loss of ink-water balance. Furthermore, when printing is conducted over a long period of time, there is a fill in of the dots of printed images. Moreover, the water allowance of the dampening system, i.e., the latitude in the graduation of the dampening solution for giving proper prints, is narrow and conducting the printing operation is difficult.
JP-A-58-176280 describes dampening solutions where certain alcohols and glycol ether esters are used as substitutive additives for IPA, and are used in combination with a water-soluble polymer. However, the problems of tinting caused by the deposition of ink on the metering roller, the fill-in of printed images, and narrow water allowances still remain.
U.S. Pat. No. 4,641,579 describes dampening solutions containing butyl cellosolve and thickening agents. However, cellosolve is a class 2 organic solvent, is harmful to the human body, can penetrate into the skin and poses problems such as nephropathy and neuropathy. Accordingly, it is not preferable to use this type of solution, even at a low concentrations over long periods of time.
JP-A-1-40393 describes a substitute dampening solution for those containing IPA. This solution can be obtained by controlling the dynamic surface tension and viscosity within certain ranges. In dampening units (a system wherein the dampening solution is carried by means of inked rollers of an ink unit, i.e., an "inker-feed dampening system") of printing presses, however, water allowance is narrow. Furthermore, roller stripping (the ink of inked rollers is peeled off) is likely.
As mentioned above, many dampening solutions have been proposed as alternatives for those containing IPA. However, those dampening solutions still have problems associated with them concerning printing performance. Suitable dampening solutions which can be used as substitutes for those containing IPA have yet to be found.